1. Field of the Invention
The present invention relates to a method of depilation of mammalian hair and also apparatus for use in the method.
2. State of the Art
The use of lasers in medical applications has been studied extensively since the early 1960's, particularly in relation to dermatology and ophthalmology. Biological tissue incorporates numerous cellular layers and different types of pigments, which respond in different ways to different types of radiation applied to the tissue. Therefore the type of laser selected for use in a particular biological application depends largely on the type of tissue to which the laser is to be applied and the nature of the effect required. Laser interactions fall generally into three distinct regimes, namely those producing photochemical, photothermal and photoionising effects. Each of these effects are caused by distinct ranges of laser parameters, such as, for example, different radiation dosages or different interaction times.
Photochemical effects are generally induced at low power output levels, and long interaction times. In contrast, the photoionising effects are generally produced with extremely high power dosages and short interaction times, usually leading to decomposition of irradiated biological material.
The use of lasers in depilation processes has been disclosed previously. For example, U.S. Pat. Nos. 3,538,919 and 4,617,926 are both concerned with depilation. These patents teach the stepwise irradiation of single hairs or hair follicles. The process described in U.S. Pat. No 3,538,919 involves inserting a laser probe within a hair follicle. The process described in U.S. Pat. No. 4,617,926 involves inserting a single hair within a bore of a fibre optic probe. These processes are time consuming, and can lead to unnecessary discomfort to a patient.
U.S. Pat. No. 5,059,192 to Zaias teaches a method of depilation using a Q-switched ruby laser, in which the laser wavelength is matched with the absorption spectrum of melanin found at the base of a hair follicle. Suitable selection of the laser parameters causes vaporization of the melanin via a single burst of energy, and thereby destruction of the selected hair follicle. Q-switched ruby lasers operate at very high power outputs for short pulse durations. A significant disadvantage in using such lasers is therefore that the apparatus cannot be used by substantially unskilled personnel because such high power is required. Such laser apparatus is therefore not suitable for cosmetic hair removal treatments. Furthermore, because Q-switched lasers operate in bursts or pulses of extremely short duration, many such bursts may be needed to cover an area of hair growth. This can be very time consuming, so that the method is not generally suitable for removing large areas of hair.
Furthermore, the use of pulsed Q-switched lasers in treatments for eradication of tattoos is known. It is well known that the irradiation of skin in this way, using Q-switched lasers, permits regrowth of hair.
It has been previously thought that the papilla was the source of life in hair and therefore that the destruction of the papilla would prevent regeneration and subsequent growth of the hair. However, it is now generally accepted that the cells in the region known as the bulge of the hair follicle in the dermis of the skin are primarily responsible for hair growth.
The dosage of laser radiation applied in the method of Zaias to destroy the papilla is between 0.4 to 10 J/cm.sup.2 in a 30-40 nanosecond pulse. This dosage is within the photoionisation regime. Such conditions are not applicable to destruction of cells of the bulge region of hair follicle which is situated in the dermis of the skin closer to the skin surface (and not in the subcutis layer, where the papillae are located to be destroyed by the method of Zaias).